1. Field of the Invention
The present invention relates to a sheet processing apparatus and, more particularly, to a sheet processing apparatus having a sheet buffer function.
2. Description of the Related Art
In recent years, in some image forming apparatus, a sheet processing apparatus is set adjacent to a paper discharge unit of an image forming apparatus main body in order to perform finishing for sheets after image formation such as sorting of the sheets or stapling the sheets.
The sheet processing apparatus has plural means for conveying a sheet, which is conveyed from the image forming apparatus, to a paper discharge tray and discharging the sheet. The means are roughly divided into a conveying path for not performing the finishing and a conveying path for performing the finishing. When the finishing is not performed, the sheet is conveyed through the conveying path for not performing the finishing and directly discharged onto the paper discharge tray. When the finishing is performed, the sheet is conveyed to a processing tray through the conveying path for performing the finishing, which is branched from the conveying path for not performing the finishing, and stacked. When a set number of sheets are stacked, the sheets are aligned on the processing tray and subjected to the finishing.
There is known a sheet processing apparatus including a paddle that drops and aligns a sheet discharged onto a processing tray. For example, the sheet processing apparatus starts driving of the paddle before a leading end of a sheet to be discharged comes into contact with stacked sheets on the processing tray, stops the driving of the paddle immediately before a trailing end of the sheet exits a conveyance path, drives the paddle again after the trailing end of the sheet reaches a position where the sheet is not kicked out by the driving of the paddle, and aligns the sheet to a predetermined position of the processing tray (JP-A-11-301912).